A high insulation treatment is performed between a coil of a motor of an electric compressor and a housing accommodating the motor so that a leakage current is not generated under a contact environment with air; however, when the compressor is in operation, the coil and the inside of the housing are exposed to a contact environment of coolant and lubricating oil. Freon-based coolant and ether-based or ester-based lubricating oil which are commonly used are dielectrics, and hence, a leakage current from the coil to the housing through the coolant or the lubricating oil is likely to be generated. Furthermore, in a process of manufacturing the electric compressor, when the motor is installed and fixed to the housing, the coil of the motor may get damaged, and this damage may cause a large leakage current. The leakage current may flow to a control unit that controls driving elements such as an engine, a brake, and a power steering through a vehicle body, adversely affect control, and lead to a serious accident by causing the driving element to malfunction. Thus, a strict reference value as a quality acceptance criterion related to a leakage current is set to a product of an electric compressor, and it is tested whether or not a leakage current of a product is the reference value or less.
A total current I flowing to the dielectric when a direct current (DC) voltage is applied to the dielectric is a grand total of a charging current Ii, an absorption current Ia, and a leakage current IL as illustrated in FIG. 5. The total current I decreases according to a change in the absorption current Ia, gradually the decrease becomes slow, decreases until influence of the absorption current Ia disappears, and finally converge to the leakage current IL. In order to measure a leakage current, it is necessary to wait until the total current converges and measure a current after converging, and it takes a long time until the total current converges. For this reason, in accurate measurement of a leakage current, a current after 10 minutes after a DC voltage is applied to the dielectric is commonly dealt as a leakage current.
A method of testing a leakage current in a short time is disclosed in Patent Document 1 is proposed. In this method, in order to test a leakage current of a capacitor, after the capacitor is charged to a setting voltage, the capacitor is charged to maintain the setting voltage, a stationary current flowing while the setting voltage is being maintained is measured, and the leakage current is measured based on the stationary current, and the leakage current can be easily tested in a short time.